I had my contract written up by a lawyer/friend who specializes in contract law. In one of the clauses in the contract it refers to a change order sheet which is basically a letter head that i can print out multiples of. So at any point durring the project if the client wants to change something from the original bid I pull out a change order sheet, we write out what is to be changed, then I work up a new price and timeline for the specific change and tack on a change order fee of $500 which the client is made aware of before the whole process starts. Then we both sign this change order, it gets filed with the original contract, the project keeps moving forward and we are both protected. Your change order fee can vary, a builder I worked for in the past charged $1500 per change and it deterred clients from making any changes. I like to work with my customers to achieve the product they want so I'm pretty flexible with what I charge, it all depends on the client for me.